This invention relates to an image processing apparatus for performing tonal and color matrix transforming on image signals. The apparatus may be applied to a color scanner system which reads image on an original, performs predetermined image processing steps and which thereafter produces a platemaking film. In this application, the apparatus is typically used to perform preliminary image processing steps such as color processing and sharpening on the input signal.
In the printing and platemaking fields, an image scanning, reading and reproducing system which produces a platemaking film by electrically processing the image information carried on an original has been extensively used with a view to streamlining the operational process and improving the image quality.
The system consists basically of an image reading apparatus which is an input machine and an image recording apparatus which is an output machine. The image reading apparatus uses typically a reading scanner which is transported in a sub-scanning direction such that the image information carried on the original is read by photoelectric scanning with a solid-state imaging device such as a CCD (charge coupled device) and converted to an electric signal. The image information thusly undergoing photoelectric conversion is sent to an associated image processing apparatus, where it is subjected to predetermined image processing in accordance with the preset conditions for platemaking. If the original image is a continuous tone image, its shades are subsequently reproduced by generating halftones to effect conversion to a halftone image, which in turn is supplied to the image recording apparatus where it is converted to an optical signal such as one of laser light and recorded on an image recording medium comprising a light-sensitive material such as a film. The image recording medium is processed photographically with a specified processor to produce a platemaking film for subsequent use as in printing.
The conventional image scanning, reading and reproducing system is such that image signals of three primary colors, such as red (R), green (G) and blue (B), are color processed for conversion to image signals of four colors Y, M, C and K, which are then subjected to resolution transformation, edge (sharpness) enhancement, tonal (gradation) transformation and other processing steps in that order, before they are finally delivered as output image signals Y, M, C and K to the image recording apparatus.
In order to reproduce an image having a greater degree of sharpness or three-dimensional effect, sharpeness or edge enhancement is performed on the image data. In the conventional sharpeness enhancement circuit, unsharp signal data are first constructed from the original image data. Then, the unsharp signal data is subtracted from the original image data to construct USM (UnSharp Mask) data. Finally, the original image data are added to the USM signal data times a constant, thereby constructing a sharpened image data. This procedure is repeated for individual monochromatic image data.
When original image data of three primary colors is to be sharpened in the prior art, said original image data is first converted to original image data of four colors including K (black) color in a color processing circuit which performs various steps such as 3-4 transformation, color correction and tonal transformation and then sharpening is carried out with the above-described sharpening circuit.
With a view to improving the conventional technology for sharpening color image data, the assignee of the subject application proposed in Japanese Patent Application No. 80733/1994 an image processing method and apparatus which are capable of freely changing the densities of respective colors, Y, M, C and K such that the degree of sharpness is adjustable to a desired level for the density of each color and which are capable of producing image data for reproducing image of reduced noise.
The proposed image processing apparatus is shown in FIG. 5, in which the apparatus is generally indicated by 80. As shown, the image processing apparatus 80 comprises: a tone transforming circuit 82 using a lookup table (LUT) for tonal transformation from 10-bit image data of three primary colors R, G and B to 8-bit image data of Y, M and C; a matrix computing circuit 84 in which a 4.times.3 matrix is multiplied by a column matrix composed of the tone transformed image data of three primaries, thereby effecting conversion to 8-bit image data of four colors Y, M, C and K; a color processing and transforming circuit 86 in which the 10-bit image data of three primaries are subjected to color processing steps such as 3-4 transformation, color correction and tonal transformation, thereby effecting conversion to 8-bit image data of four colors Y, M, C and K; and a sharpening circuit 92 consisting of a USM signal data constructing circuit 88 which receives the four-color image data from the matrix computing circuit 84 to construct USM signal data for each color and an adder circuit 90 which, for each color, adds the 4-color USM signal data to the 4-color image data delivered from the color processing and transforming circuit 86.
The LUT 82 stores 8-bit image data for each of the three colors Y, M and C, such as Yj (j=0-255), Mj (j=0-255) and Cj (j=0-255), that have been obtained by performing a specified procedure of tonal transformation on all of the 10-bit image data for each of the three primary colors, say, R, G and B, namely, Ri (i=0-1023), Gi (i=0-1023) and Bi (i=0-1023). For example, LUT 82 is such that the tone transformed image data that correspond to image data Ri (i=0-1023) are written at addresses (0,0)-(0,1023), data corresponding to Gi (i=0-1023) are written at addresses (1,0)-(1,1023), and data corresponding to Bi (i=0-1023) are written at addresses (2,0)-(2,1023); the LUT 82 outputs 8-bit tone transformed image data (Yj, Mj, Cj) that have been read in response to the 10-bit input image data (Ri, Gi, Bi).
In the matrix computing circuit 84 at the next stage, a 4.times.3 matrix is multiplied by a column matrix of input image data (Yin, Min, Cin) to deliver an output (Yout, Mout, Cout, Kout) in accordance with the following formula (I): ##EQU1##
A problem with the image processing apparatus 80 shown in FIG. 5 is that in order to perform tonal transformations and matrix calculations as pre-sharpening steps, tone transforming LUT 82 and matrix computing circuit 84 are respectively required and this adds to the scale of the overall circuit. As a further problem, matrix computing circuit 84 requires a number of multipliers, which adds to the cost of the circuit.
Another problem concerns the 3-4 transformation for transforming image data of three primary colors to image data of four colors. Since the first step of this process is tonal transformation from 10-bit image data to 8-bit data, image data of the fourth color, namely, K (black) color, will be subjected to matrix calculations on the basis of tone transformed image data of three primary colors such as Y, M and C and the intended matrix computation cannot be performed independently of those data of three primary colors. In other words, independent tone transformed data cannot be constructed for the fourth (i.e., black) color.